What's Mean Lighten up?

intransitive verb. : to take things less seriously.

What does it mean by lighten up?

Become or cause to become less serious or gloomy, and more cheerful. For example, Lighten up, Sam—it'll turn out all right. This slangy expression transfers reducing a physical weight to a change of mood or attitude.

Where does the phrase lighten up come from?

Its origin can be based on a legend that says that when people die their heart is weighed to check whether they have performed good or bad deeds, if it is heavy they will straight away go to hell and if it is light, they will enter heaven.